Establishment of an emerging generalist pathogen in redwood forest communities

Abstract

Summary1 Phytophthora ramorum (causal agent of sudden oak death) is an emerging generalist pathogen in coastal forests of California and Oregon, USA, that causes lethal stem infections on oaks ( Quercus spp.) and tanoak ( Lithocarpus densiflorus ) as well as nonlethal foliar infections on a broad range of trees and shrubs. 2 We studied P. ramorum over its known range in coastal redwood forests to determine forest compositional variables that are important to its epidemiology within the geographical area that it has already invaded. Redwood forests are dominated by coast redwood ( Sequoia sempervirens ), tanoak and California bay laurel ( Umbellularia californica ). 3 A total of 120 permanent plots (500 m 2 each) were established in redwood forests at 12 sites within the main epidemic area in California. Over 5000 trees were mapped and examined for the presence of P. ramorum during spring 2002 and resampled in spring 2003. 4 Mean incidence of P. ramorum across all plots was 0.17 ± 0.01 in 2002 and 0.24 ± 0.02 in 2003. The highest infection levels by P. ramorum were found on California bay laurel (range 0.42-0.69) and tanoak (0.32-0.45). The highest levels of mortality were associated with tanoak and ranged from 0 to 66%, with 62.5% of that mortality associated with P. ramorum infection. 5 Disease incidence above 30% was most often associated with bay laurel importance value. In plots with few bay laurel stems, high disease levels were associated with the presence of understorey tanoaks. Bay laurel and small tanoaks are thought to represent the main source of inoculum for further spread of P. ramorum . 6 Differential host mortality due to this emerging generalist pathogen will exert considerable influence on redwood forest dynamics, with potentially dramatic shifts in forest composition and structure and subsequent cascading ecological and evolutionary effects.

The influences of forest stand management on biotic and abiotic risks of damage

“…Hylobius abietis Björklund et al, 2003) where they can hide and repeatedly return to feed on seedlings, thereby causing serious damage. For several pathogenic fungi such as Phytophthora, the understorey vegetation may also serve as a reservoir thus increasing the inoculum pressure on neighbouring trees or seedlings (Brasier et al, 2005;Maloney et al, 2005;O'Hanlon-Manners and Kotanen, 2004). Some heteroecious fungal pathogens, such as the rust disease Melampsora pinitorqua, need to alternate between two different host plants, one of which is often present in the understorey vegetation.…”

A latitudinal cline in disease resistance of a host tree

“…Further examples exist as the result of the introduction of pathogens into ecosystems. These include the impact of Asian chestnut disease (Cryphonectria parasitica) on forests formally dominated by American chestnut (Castanea dentata) (Stephenson, 1986), and the impact of the invasive generalist pathogen Phytophthora ramorum, the causal agent of sudden oak death, on the composition and structure of coastal forests in California and Oregon (Maloney et al, 2005).…”

Optimization of control strategies for epidemics in heterogeneous populations with symmetric and asymmetric transmission

“…The pathogen has a wide host range and is currently A c c e p t e d m a n u s c r i p t spreading rapidly through coastal regions of California (Rizzo et al, 2002. Here we focus initially on spread through communities dominated by two major host species with asymmetrical transmission between bay laurel (Umbellularia californica) and tanaok (Lithocarpus densiflorus) (Maloney et al, 2005). Rowthorn et al (2009) recently analysed optimal strategies for the deployment of disease control on a single host species comprising two or more spatiallyseparated sub-populations in which infected individuals recover and can be reinfected.…”